1. Field of the Invention
The present invention relates generally to an operation lever unit for engine-powered working machines, and more particularly to a throttle lever unit provided on a rod of a carrying bush cutter having a circular cutter driven in rotation by an engine carried on the back of an operator.
2. Description of the Related Art
A carrying bush cutter having a circular cutter attached to the top of a hand-operating rod and driven in rotation by an engine carried on the back of an operator for achieving a bush-removing work is disclosed in, for example, Japanese Utility Model Publication No. SHO 63-14035. In use of the disclosed bush cutter, the operator swings the rod in vertical and horizontal directions while gripping an operation handle provided on the rod, so as to remove bushes by the rotating circular cutter. In order to control the rotational speed of the cutter, output power of the engine is regulated by a throttle lever provided on a grip portion of the operation handle.
Disadvantageously, because the operator is forced to continue gripping of the throttle lever together with the operation handle throughout a bush-removing work, a heavy work load is put on the operator.
Somewhat successful prior improvements have proposed a lock mechanism associated with the throttle lever to lock the throttle lever in a desired position to thereby reduce the work load on the operator. Typical examples of the prior improvements are disclosed in Japanese Utility Model Publications Nos. SHO 53-42661, SHO 55-21536 and SHO 60-41539.
According to the disclosed operating lever units, the throttle lever is displaced to a predetermined operating position, then locked in this operating position by activating the lock mechanism. The lock mechanism is released at need.
More specifically, the operation lever unit disclosed in Japanese Utility Model Publication No. SHO 55-21536 is constructed such that when a throttle adjustment is needed, the throttle lever is turned by the thumb or the second finger of a hand of the operator while the operation handle and an unlock lever are continuously gripped by the same hand. Actuation of the throttle lever using a single finger is unable to guarantee smooth and reliable movement of the throttle lever in both the throttle-opening direction and the throttle-closing direction.
The operation lever unit disclosed in Japanese Utility Model Publication No. SHO 60-41539 is designed such that when a throttle adjustment is needed, a lock lever is turned to a given position by using the second finger of a hand of the operator and the throttle lever is turned to a desired position by the second finger. During that time, the operation handle is continuously gripped by the same hand. Actuation of the two levers using a single finger is insufficient to provide a smooth and reliable movement of the throttle lever in both the throttle-opening direction and the throttle-closing direction.
The operation lever unit disclosed in Japanese Utility Model Publication No. SHO 53-42661 is designed such that when a throttle adjustment is needed, a throttle trigger is actuated by the thumb while gripping the operation handle together with a safety tripper. Likewise the operation lever units shown in Japanese Utility Model Publication Nos. SHO 55-21536 and SHO 60-41539, the throttle adjustment disclosed in this Japanese Publication relies on the use of a single finger. Accordingly, a precise throttle adjustment is difficult to achieve.
In general, the bush cutter while in use for bush-removing operation is subjected to various sorts of vibrations caused due, for example, to running of the engine, rotation of a drive shaft extending through the operating rod, and rotation of the circular cutter. In order to keep a satisfactory level of working efficiency, the operator is required to continue a firm grip on the operation handle throughout the bush-removing work.